Download Astrophotography

Autoguiding
Beginners Guide To
Autoguiding Basics
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What is Guiding?
Background
Why is Guiding Required?
Technical Blurb
Autoguider Setups
How it Works
Autoguiding Accuracy
What I Use
PHD Guiding
Conclusion/Tips/Closing
What is Guiding?
For an astrophotography image to be clear the target must be held in a fixed position
within the telescope's field of view; any movement of the scope during an exposure
would cause point sources of light to appear as streaks.
• Guiding is the process of assisting the telescope mount to keep a celestial body
exactly centred while taking long exposure images. And is usually required when
imaging dim Astronomical targets where long exposure times are necessary (e.g.
30 Seconds or more).
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Manual Guiding is the process of a person watching a guide star and manually
adjusting the mount during long exposures.
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Autoguiding is an electronic tool or system used to automatically keep the celestial
body being photographed from drifting across the field of view during long
exposures.
Background
Before the availability of cheap and powerful Digital cameras and computers, guiding
was accomplished by manually adjusting the orientation of an equatorial mount to
hold either the target itself or a nearby guide star at a certain position. This method
was:
• time consuming.
• difficult to carry out over long periods.
• prone to error (it took time for the operator to see the movement and react to it).
• If the operator pressed the wrong direction key it would ruin the whole image.
• Not very accurate.
Background con’t
Furthermore before the availability of cheap and powerful CCD and CMOS imaging
devices people used film for their photo’s, film is prone to a few issues when used for
Astrophotography:
• temperature quickly affected the sensitivity of the film (and is usually shot in cold
locations at night).
• film suffers from a phenomenon called reciprocity (false colour shift).
• you cannot instantly see your results with film so hours of work manually guiding
may have been for nothing at the end of the day.
• you were not able to stack or digitally manipulate your images to enhance the
target objects like we can with digital media today.
Why is Guiding Required?
Astrophotography demands an extremely high level of precision that even modern
computer-tracked mounts and GoTo telescopes cannot achieve.
Even “the best” mounts, made with high tolerances, that are accurately polar aligned
and running Periodic Error Correction programs cannot achieve the level of accuracy
required and will show some level of error in the image being taken. The errors are the
result of imperfections in:
• the drive gears
• in other areas of machining of the mount
• play in the ball bearings that permits their rotation
• Movement or Flexing in mount and brackets
• Misalignment from the celestial pole
Why is Guiding Required? con’t
Since even “the best” mounts have these problems to some extent, affordable
beginner-class mounts certainly have it. However don’t lose heart, there are ways
around these issues and the use of an Autoguider definitely makes this easier.
Here is an example of the difference between a non-guided and a guided image.
Technical Blurb
Modern telescope mounts use a control system called an “open loop” controller (also
called a non-feedback controller) this is a type of controller that computes its input
into a system using only the current state and its model of the system. A characteristic
of the open-loop controller is that it does not use feedback to determine if its output
has achieved the desired goal of the input.
This means that the system does not observe the
output of the processes that it is controlling.
Consequently, an open loop system cannot correct any
errors that it could make and it cannot compensate for
disturbances in the system.
Open loop controllers are chosen as they are simpler to
manufacture and therefore much cheaper to make and
subsequently cheaper to purchase.
Technical Blurb con’t
To avoid the problems of open-loop controllers, a feedback circuit can be introduced.
The feedback circuit effectively closes the loop and creates a closed-loop controller.
Closed loop controllers have the following advantages over open loop controllers:
• disturbance rejection (such as unmeasured friction in a motor).
• guaranteed performance even with model uncertainties, when the model
structure does not match perfectly the real process and the model parameters are
not exact (e.g. poor polar alignment)
• unstable processes can be stabilized
• reduced sensitivity to parameter variations
• improved reference tracking performance
Autoguiding is for all intents and purposes a system that provides the components
required to create a closed loop system and provides the above benefits.
Autoguider Setups
An Autoguider set up comprises the following basic configurations:
• Guide Scope and Camera - a small telescope oriented in the same direction as the
main telescope, (Cheapest and most common setup).
• Off-axis guider – mounted on the main scope that diverts some of the light
originally headed towards the Imaging device to the guide camera.
• On-Camera guider – An imaging camera with an integrated guiding chip.
Auto Guider Setups con’t
All Autoguider setups have a camera that watches a guide star and checks for
apparent movement. These cameras come in a wide variety of configurations from a:
• Web Cam (usually modified to attach to a scope and see fainter images)
• Dedicated Guiding Cameras (Such as the Orion Star Shoot Autoguider
• Astronomical Video Cameras (Such as the GStar EX Range)
Auto Guider Setups con’t
This image is my set up in a side by side configuration,
the scope with the black cover is the guide scope.
I use the Orion Star Shot Autoguider Camera that
connects to the Laptop via the USB port and to the
mount via the ST4 Cable.
Auto Guider Setups con’t
This image is a schematic of the connections for the Orion Star Shoot Guide Camera.
How it Works
The guide camera is a simple CCD sensor that regularly takes short exposures of an
area of sky near the object being imaged. After each image is captured, its controller
measures the apparent motion of one or more stars within it’s view and issues the
appropriate corrections to the telescope's (computerized) mount.
Autoguiders require a controller to calculate
the changes in apparent motion of the guide
stars, these can be either a dedicated
hardware based platform (Such as Sbig’s ST4)
or via a PC with a guiding program such as
PHD Guiding, Guidemaster, Guidedog, or
general-purpose astronomical software such
as MaxDSLR.
Autoguiding Accuracy
Why is Autoguiding better than manual guiding? Simply put the guide star illuminates
more than one pixel on the guide camera, as such Autoguiders are able to use the
variations of light falling on each pixel to calculate where the star should actually be
located.
As a result, most Autoguiders have sub pixel accuracy. In other words, the star can be
tracked to an accuracy better than the angular size represented by one CCD pixel.
However due to affects such as atmospheric turbulence, the typical limit of accuracy is
usually down to one arc second.
An auto-guider can't make up for large errors in alignment, extremely poor mount
performance, field rotation, or other gross errors in tracking.
What I Use
I personally use the Orion Star Shot Autoguider
package and PHD with a Laptop. PHD is free,
well documented and support is very easy to
get from other users on the Internet. It also
fully supports the ASCOM platform.
Oh and did I mention it’s free?
PHD literally stand for:
Press
Here
Dummy
So it is easy to use.
PHD Guiding
This is an image of PHD
running and focused on a
guide star. You can see in the
graph on the right the level
of the commands sent to the
mount to correct for any
variation in the guide star.
PHD is as simple as:
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Chose your camera
Select your telescope
Start the image
Select your guide star
Click PHD button
Wait to calibrate and start guiding
Conclusion
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Autoguiding is an automated way of stabilizing and regulating the
tracking performance of your telescope mount.
Autoguiding has reduced the manual effort required when taking
Astrophotos.
Autoguiding will measure and correct for the inconsistencies in
you mount to ensure it tracks the motion of the night sky
smoothly.
There is a lot of good and free software packages available for use
to control your guiding set up.
Autoguider setups come in a wide variety of flavors and costs, so
find the one that fits your requirements.
Autoguiding is very precise and can measure sub pixel variations
in movement and measures the changes very frequently.
Tips
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Autoguiding does have it’s limitations and is not a fully closed loop
system, as such it is:
– Still important to make sure you have setup, balanced and
aligned your mount correctly.
– Your mount is stable and does not have any significant
malfunctions (such as excessive play in bearings or gears).
– Autoguiding require a stable and rigid mount so there is no
flexing between the guider and the main scope.
Ask people already doing AP for advice and guidance on how to
use Autoguiders, There is a wealth of knowledge out there.
Research before buying things to make sure you understand how
they work and that it will work for your intended purpose and
equipment.
Test your equipment at home in the backyard before you go out to
the Dark Sky site to avoid problems in the field.
Closing
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As with all thing astronomy share your ideas and findings with
others to help them along their journey
Remember you don’t need to spend a fortune to get a guiding
system running as there are many tools that are very cheap or
free to use, such as PHD.
Please support these developers by making a donation if you can
afford to, even $5 could ensure they are around tomorrow
providing free software.
Pray for clear skies 
Any questions???